I. Field of Invention
The present invention relates to a method of crystallizing a silicon film and a method of manufacturing a liquid crystal display apparatus which uses the Joule heat of a heat generating conductive layer to increase the temperature of a silicon film for expediting silicon crystallization.
II. Discussion of Related Art
An active layer of a thin film transistor (hereinafter abbreviated TFT) in a liquid crystal display (hereinafter abbreviated LCD) is made of a silicon film which is in a polycrystalline state because the mobility of electric charges of polycrystalline silicon is higher than that of amorphous silicon. Polycrystalline silicon has usually been formed at a high temperature. A new technique of manufacturing a TFT of polycrystalline silicon at low temperature is currently being used.
Low temperature polycrystalline silicon has various advantages such as the low process temperature, the capability of forming a large LCD area and the comparability of performance equal to the high temperature polycrystalline silicon process. There are several methods of forming low temperature polycrystalline silicon such as Solid Phase Crystallization (hereinafter abbreviated SPC), Laser Crystallization and other methods.
Laser crystallization crystallizes amorphous silicon into polycrystalline silicon by applying a laser to an amorphous silicon layer under 400 degrees Celsius and provides excellent performance. Unfortunately, the crystallization fails to provide uniformity. The method is not suitable for manufacturing polycrystalline silicon for a large scale LCD due to the high cost of equipment and low productivity.
SPC crystallizes amorphous silicon into the polycrystalline silicon (polysilicon) by carrying out heat treatment at 550 to 700 degrees Celsius for about 24 hours and provides uniform polycrystals using equipment that is not so expensive. Unfortunately, the temperature and time required for crystallization are high and long so that a glass substrate cannot be used. Also, productivity using this method is low.
Another technique for the crystallization of amorphous silicon is Metal Induced Crystallization (hereinafter abbreviated MIC) which is illustrated in FIG. 1A and FIG. 1B. MIC achieves crystallization by contacting amorphous silicon with a metal catalyst which accelerates the silicon crystallization at about 500 degrees Celsius.
Referring to FIG. 1A, after a buffer layer 10 of silicon oxide has been formed on an insulated substrate 100, an amorphous silicon layer 11 is deposited on the buffer layer 10. Then, a metal film 13 such as a Ni film working as a catalyst layer for crystallization is formed on the amorphous silicon layer 11. In this case, the metal film 13 of Ni is deposited on the amorphous silicon layer 11 by sputtering which is a conventional method of depositing metal.
Referring to FIG. 1B, the amorphous silicon layer 11 undergoes heat treatment on the above substrate for crystallization.
As a result of the heat treatment, silicide (not shown) is formed by diffusion of the Ni layer toward a silicon layer so as to form a silicide region at the boundary of the silicon layer and Ni layer. The silicide accelerates the crystallization of the silicon film to crystallize the amorphous silicon layer into a polycrystalline film 19 at a low crystallization temperature.
In MIC as a related art, silicon crystallization occurs by forming a Ni film having a predetermined thickness. Thus, excessive Ni in an amount equal to the thickness of the metal layer remains in the crystallized silicon film. Therefore, a TFT of polycrystalline silicon contaminated with the excessive Ni is unable to be used as a switching device due to degraded device characteristics.
Moreover, it takes more than 10 hours to crystallize silicon and the crystallization temperature is not that low. Thus, the time and temperature required for this process are unacceptable.
Metal induced lateral crystallization [hereinafter abbreviated MILC, S. W. Lee & S. K. Joo, IEEE Electron Device Lett., 17(4), P.160, (1996)] as an alternative method has been proposed lately.
MILC as shown in FIG. 2 is performed such that silicon crystallization is induced laterally in a predetermined region which has been crystallized by MIC. In MILC, a portion of amorphous silicon 22-1 contacted with a specific metal 23 is crystallized by MIC, and a boundary of the crystallized silicon region 22-1 becomes a seed for crystallizing laterally the adjacent portion of the amorphous silicon 22-2 which is not directly contacted with the metal. Unfortunately, the crystallization speed of MILC is slow.